The use of membrane introduction mass spectrometry (MIMS) as an online measurement strategy for volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs) in air [1], water [2] and complex reaction media [3-7] has been well demonstrated over the past decade. The method provides continuous, direct introduction of sample to a mass spectrometric system without sample preparation or pretreatment steps and has been applied as a “real-time” monitoring strategy for dynamic chemical processes and environmental systems [3-10]. The theory and practice of MIMS is discussed in several recent reviews [11, 12]. The membrane sampling interface (typically a polydimethylsiloxane (PDMS) membrane) acts as a semi-selective barrier that rejects the bulk of the sample matrix, while allowing the permeation of VOC and SVOC analyte molecules across the membrane. Analytes are subsequently entrained by an inert sweep gas that is introduced directly into a mass spectrometer [13]. The principle advantages of membrane introduction are the ability to monitor selected analytes in complex matrices in an on-line system with temporal resolutions on the order of seconds to minutes. The sensitivity of MS detectors in general and the added selectivity afforded by tandem MS techniques makes MIMS a powerful analytical tool. MIMS is well suited for the analysis of non-polar, low molecular weight analytes, providing low detection limits (<ppb) and fast instrument response times (<1 min). However, the analysis of SVOCs by conventional MIMS provides unsatisfactory results characterized by slower response times and relatively high detection limits (when compared to VOCs).
The overall permeation of analyte across the membrane has been described as i) selective absorption into the membrane, ii) diffusion through the membrane itself and iii) desorption from the downstream membrane surface into the carrier gas [12, 13]. The permeation rate of analyte through the membrane and hence the response time of the measurement process is governed by physio-chemical properties of the permeating species and the concentration gradient across the membrane. In addition, both theory and experiment have shown the permeation rates are determined by properties of the membrane, including its composition, thickness, surface area and temperature. Several papers have appeared evaluating a variety of membrane geometries [3, 13-17] and membrane materials [14]. However, the greatest improvements in extending MIMS to less volatile molecules involve cryo-focusing techniques [18] or some form of thermally assisted desorption, typically involving heating the entire MIMS interface assembly [19-21]. For example, Lauritsen [22] and Eberlin [23] have reported on methods in which sample is passed through the lumen of a hollow fibre membrane positioned in the EI mass spectrometer source. Heating the membrane by the EI filament led to greater analyte desorption and increased sensitivity. Soni et al. described the use of a low-power carbon dioxide laser to desorb analyte from a sheet membrane directly into a mass spectrometer source [24] and recently, Creaser et al. have developed a “universal MIMS interface” that incorporates heating and cooling to facilitate both VOC and SVOC measurements [25].
Thermal desorption methods have been successful in improving the analytical performance of MIMS to the analysis of SVOCs in air samples [25, 26]. By elevating the interface temperature after pre-loading the membrane with analyte, SVOCs permeate and desorb more rapidly. Thermal desorption methods for aqueous samples (in which both membrane and sample are heated) have been limited by much slower heating cycles and broadened desorption profiles, due to the relatively high heat capacity of water. This is further complicated by the increased permeability of the membrane to water vapour at increased temperatures. During the online analysis of SVOCs in aqueous samples the MIMS interface is typically heated to a maximum of 50-60° C. At higher temperatures, sufficient water vapour is transferred through the membrane to the mass spectrometer that the overall MS performance is degraded [27]. Furthermore, the partitioning of analyte into the PDMS membrane is less favored at elevated temperatures thus reducing the analytical sensitivity. These limitations can be circumvented by using a ‘trap and release’ approach in which the aqueous sample is exposed to the membrane and then removed, followed by a more rapid thermal desorption of (SVOC) analytes ‘trapped’ in the membrane [21]. However this approach comes at the expense of continuous (real-time) measurement capability. Riter et al. have demonstrated a ‘single-sided MIMS’ device which uses a double helical wire heater coil inside a membrane to desorb material from the inner surface of a membrane [28]. This device operates by exposing the sample to the same (inner) membrane surface that is ultimately exposed to a mass spectrometer (after a pump down cycle). Such methodology, although well suited for new membrane materials development research or as a pre-concentrator for fiber introduction mass spectrometry [29], does not facilitate ‘real-time’, temporally resolved measurements. In recent work by the Thomas group, an internally heated HFM-PDMS membrane is modeled and demonstrated as a means of effecting low resolution separations for ppm level VOC analytes using flame ionization detection [30]. Their work suggests that internally heated membrane capillaries can be used for the low resolution separation of analytes, and may prove useful for mixture analysis.
It is an object of the present technology to overcome the deficiencies in the prior art.